Systems for determining the intra-arterial blood pressure of a patient can be subdivided into two main groups-those which invade the arterial wall to directly access blood pressure and those which use non invasive techniques. For a long period of time, the most accurate blood pressure measurements were achievable only by use of invasive methods. One such common method involved use of a fluid filled catheter inserted into the patient's artery.
While invasive methods provide for accurate blood pressure measurements, the risk of infection and potential for complications, in many cases, outweigh the advantages of using invasive methods. Because of the risk of complication associated with invasive methods, a noninvasive method, known as the Korotkoff method is widely used. The Korotkoff method is known as an auscultatory method because it uses the characteristic sound made as the blood flows through the artery to denote the high and low blood pressure points. Although the Korotkoff method is noninvasive, it only provides a measurement of the highest blood pressure (systolic) and the lowest blood pressure (diastolic) along the continuous pressure wave form. While systolic and diastolic pressure are often sufficient for accurate diagnosis, there are many applications in which it is desirable to use the entire curve of the blood pressure wave form. In these applications, the Korotkoff method simply is incapable of providing satisfactory information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion of the artery in which blood pressure is being monitored. While arterial occlusion is not prohibited in many applications, there are occasions where the patient's blood pressure must be monitored continuously (such as when undergoing surgery) and accordingly, prohibiting blood flow, even on a temporary basis, is undesirable or unacceptable. Other problems associated with the Korotkoff method include the fact that the cuff must be properly sized with respect to the patient and the detrimental affects of respiration and acoustic noise on overall measurement accuracy.
Because of the above mentioned risks involved with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, noninvasive blood pressure monitoring and recording methods. Many of these noninvasive techniques make use of tonometric principles which center around the fact that as blood flows through the arterial vessel, forces are transmitted through the artery wall and through the surrounding arterial tissue and are accessible for monitoring. Because the tonometric method of determining blood pressure is noninvasive, it is used without the risks associated with invasive techniques. Furthermore, since it does not suffer from the limitations of the auscultatory method, it has the capability of reproducing the entire blood pressure wave form, as opposed to the limited systolic and diastolic pressure points provided by the Korotkoff method.
In several of the prior art arterial tonometers, a row of individual transducer elements, such as strain gauges or the like, are placed in direct contact with the tissue which overlays an arterial vessel from which blood pressure is to be measured. As the blood pressure within the arterial vessel increases and decreases the vessel wall expands and contracts thereby transmitting forces through the overlying tissue and onto the row of transducer elements. Although the individual elements are dimensionally sized so that several are required to cover the entire diameter of the underlying arterial vessel, their discrete character prevents reconstructing a true continuous contour of the tissue stresses which occur across the entire row of elements.
It has also been found that many prior art tonometry sensors are cumbersome, difficult to administer and uncomfortable to wear for any long period of time.
Thus, it is desirable to provide a noninvasive tonometry system for determining the blood pressure in an arterial vessel by measuring the stress of the tissue overlaying the arterial vessel.
Still further, it is desirable to have a system which is capable of accurately reconstructing a continuous stress contour across the diameter of an artery of interest.
It is also desirable to have a system which automatically compensates for errors introduced into the tissue stress signal which result from temperature, aging or other factors which influence the tissue stress sensor.
Additionally, it is desirable to have a miniaturized sensor which can be easily administered and comfortably worn for long periods of time.